Multi-track off-road vehicles such as dune buggies (also known as sandrail vehicles) and smaller all-terrain vehicles (ATVs) having more than one wheel track are subject to loss of steering traction when driving over loose terrain such as sand or loose soil. Steering traction may be further compromised when driving at high speeds or when accelerating. Dune buggies are especially prone to loss of steering traction because they typically have very little weight supported by their front wheels and their front ends tend to lift when accelerating. Loss of steering traction can result in substantial understeer or, in severe cases, a total loss of steering, thereby decreasing maneuverability and increasing the risk of accidents. To compensate for loss of steering traction, some dune buggies have lever-operated turn brakes that allow the driver to turn the dune buggy by manually applying differential braking force to the rear wheels.
FIG. 1 shows a hydraulic cylinder assembly 10 used in prior-art lever-operated turn brakes. With reference to FIG. 1, a pair of hydraulic cylinders 11, 12 are spaced apart within a housing 13 that includes mounting tabs 14 for attaching the hydraulic cylinder assembly 10 to a frame of a dune buggy (not shown). A first hydraulic line 15 couples the first cylinder 11 to the braking mechanism of the dune buggy's right rear wheel (not shown), while a second hydraulic line 16 couples the second cylinder 12 to the left rear wheel (not shown). A hydraulic input line 17 supplies hydraulic brake fluid from a foot-brake master cylinder reservoir (not shown) to the cylinders 11, 12. A rocker lever 18 having a handle 19 is coupled to the cylinders 11, 12 and pivots about a pivot point 20 located medially of the cylinders so that moving the handle 19 selectively applies pressure to one cylinder while releasing pressure on the other cylinder. In this manner, the dune buggy can be steered over loose terrain merely by manually moving the rocker lever 18. The hydraulic cylinder assembly 10 would typically be located adjacent the driver's seat of the dune buggy, within the driver's reach, typically near the transmission shift lever. A pedal-operated hydraulic main brake typically operates in conjunction with the turn brake system for stopping the dune buggy.
Independently operated pedal brakes having two pedals, one for each of the left and right brakes, are also known for use in tractors to improve maneuverability, as described, for example, in U.S. Pat. No. 5,022,477 of Wanie.
Both of these prior-art systems require the driver to provide manual inputs different from the familiar motions used in driving an automobile. Lever-operated turn brakes require the user to take one hand off the steering wheel and reach for a lever at a time when he or she may have an increased need to keep both hands on the steering wheel for maneuvering purposes. The driver may also need to look away from the direction in which he or she is driving in order to find the handle of the turn brake. Similar problems can arise with pedal-operated turn brakes, which require at least one foot to operate.
Many other known systems for differential braking include a distributor valve or a pair of valves that are mechanically coupled to the steering wheel for releasing compressed air or hydraulic fluid to the left and right wheel brakes selectively, in response to turning of the steering wheel, as described in U.S. Pat. Nos. 2,254,902 and 2,366,207 of Milster, U.S. Pat. No. 2,360,843 of Boldt, U.S. Pat. No. 2,442,601 of House et al., U.S. Pat. No. 2,474,961 of Sneed, U.S. Pat. No. 3,129,035 of Alfieri, U.S. Pat. No. 3,603,424 of Blood et al., and U.S. Pat. No. 3,877,537 of Ohms et al. These valve-actuated systems require a source of pressurized operating fluid, such as pumped hydraulic fluid or compressed air. In the systems described in U.S. Pat. Nos. 2,254,902, 2,366,207, 2,442,601, and 2,474,961, the fluid pressure is generated by pumping a foot brake pedal at the same time as the valves are actuated via the steering wheel. In the systems of U.S. Pat. Nos. 2,360,843, 3,129,035, 3,603,424, and 3,877,537, fluid pressure is generated by a powered compressor or hydraulic pump. Both types of systems require braking force to be generated by a source other than the torque being applied to the steering wheel.
U.S. Pat. No. 3,893,528 of Rehfeld describes a system for temporarily applying differential braking force to the front wheels of an automobile that includes a hydraulic piston assembly integrated with the steering column of a power-assisted steering system. Differential braking force is generated by a piston that is driven linearly only when steering torque exceeds a predetermined threshold, e.g., in response to a loss of hydraulic pressure in the power-assisted steering system or when turning sharply in an emergency maneuver. Because steering torque does not exceed the threshold during normal steering maneuvers and because the differential braking force is applied only to the front wheels, the system of Rehfeld would be inoperable in the event of a loss of front-wheel traction, for example when driving over loose soil or sand.
The inventor has recognized a need for an improved supplemental steering system for a multi-track vehicle that generates differential braking force in response to turning of the vehicle's steering wheel and that can compensate for a loss of front-wheel steering traction.